Fuse structures for elevated circuit voltages



Dec. 3, 1963 F. J. KozAcKA 3,113,195

FUSE STRUCTURES FOR ELEVATED CIRCUI'YI` VOLTAGES Filed Feb. 5, 1962 2 Sheets-Sheet 1 4 I INVENTOR.

FREDERICK J. KOZACKA ,KJ/MW ATTY.

Dec. 3, 1963 F. J. KozAcKA 3,113,195

FUSE STRUCTURES FOR ELEVATED CIRCUIT VOLTAGES Filed Feb, 5, 1962 2 sheets-sheet 2 m 600 l g /f/ 'I Av2 J /IF f zoo l i Av,

1 l l l o u /1 i '1r l/ o i l FIG. 7 l; M :l INVENTOR FREDERICK J. KOZACKA JAMW M/WTTY.

United States Patent O 3,1l3,l95 FUSE STRUCTURES FR ELEVATED CHClUIT WUI/PAGES Frederick J. Kozaclka, South Hampton, NH., assigner to The Chase-Shawmut Company, Newburyport, Mass. Filed Feb. 5, 1962, Ser. No. 171,058 Claims. (Cl. 20d- 131) This inventinon relates to electric fuses, and more particularly to current-limiting fuses, briey referred to as CL fuses.

It is one object of this invention to provide improved CL fuses for 'voltages above `600 volts which are compact and adapted to carry relatively high currents.

It is another object of this invention to provide CL fuses which are easier and less expensive to manufacture than those disclosed in U.S. Patent 2,647,970 to W. S. Edsall et al., Current-Limiting Protective Device, Aug. 4, 1953, yet perform in a fashion similar to that in which fuses according to the aforementioned patent perform.

Another object of the invention is to provide fuses involving the principle of stabilization of the arc voltage disclosed and claimed in U.S. Patent 2,964,604 to P. C. Jacobs, Jr. et al., CurrentLimiting Fuses Having Compound Arc-Voltage Generating Means, Dec. 13, i960, yet including means tending to quench more effectively arcs formed incident to interruption of electric circuits.

This application is a continuation-in-part of my copending patent application Serial No. 775, 881; Fuse Structures; tiled Nov. 24, 1958; now U.S. Patent 3,020,372; issued Feb. 6, 1962. That patent shows a way of increasing the time-lag of time lag fuses by means of pairs of insulating plates sandwiching portions of a fuse link and exposing other portions thereof to the action of a pulverulent arc-quenching filler in which the fuse link is submersed.

It is one object of this invention to provide CL fuses achieving increased time lags by the same means, and in the same fashion, as in structures embodying the teachings of the aforementioned US. patent, yet including means tending to quench more effectively arcs formed incident to interruption of electric circuits and, therefore, lending themselves particularly well for application in the range of voltages above 600 volts, up to several kilovolts.

My invention will be better understood and its various objects and advantages will be more fully appreciated from the following description taken in conjunction with the accompanying drawings in which:

FIG. l is in substance a section taken along ll-l of FIG. 2 and showing a fuse embodying my invention intended to be applied in circuits having a circuit voltage in the order of -a few kilovolts;

FIG. 2 is in substance a section taken along 2-2 of FIG. l;

FIG. 3 is in substance a section taken along 3 3 of FIG. 4 showing another fuse embodying my invention intended to be applied in circuits having a circuit voltage in the order of 1000 volts;

FIG. 4 is in substance a section taken along 4 4 of FIG. 3;

FIG. 5 shows a portion of FIG. 1 on a larger scale;

FIG. 6 shows a portion of FIG. 2 on a larger scale; and

FIG. 7 shows the voltage distribution along an arc gap formed in a prior art fuse at two different points of time subsequent to blowing of the fuse.

Referring now to the drawings, and more particularly to FIGS. 1 land 2 thereof, reference numeral r1 has been applied to indicate a tubular casing of insulating material containing a pulyerulent arc-quenching lfiller 2. Filler 2 is a substance having a relatively high de-ionizing action, and is preferably quartz sand. A pair of terminal elements or terminal caps 3 closes the ends of casing ll.

ICC

Terminal elements or caps 3 are conductively interconnected by a ribbon fuse link 4 of a relatively high fusing point metal defining serially related points of reduced cross-section 4a. Fuse link 4 consists preferably of silver, or of copper, or of an alloy having performance characteristics similar to those of silver and copper. Each of the aforementioned points of reduced cross-section 4a is formed by a circular perforation and two semi-circular perforations. The axially outer ends of ribbon link 4 project through holes or apertures 3a in caps 3 and are conductively connected to caps 3 by solder joints 5. A preponderant portion of the entire llength of link 4 is sandwiched between a pair of plates `6 of insulating material evolving gas under the heat of electric arcs. Plates 6 are preferably made of a synthetic-resin-glass-cloth laminate, e.g. a melamine-glass-cloth laminate. Link 4 may be tinplated, the tin plating being in order of JA000. Another plating of a low fusing point metal acting substantially in the same way las a tin plating, i.e. causing link 4 to be severed by a metallurgical reaction when the fusing point of the low fusing point metal is reached, may be substituted for the plating of tin. Such low-fusing-point metals are, for instance, cadmium and indium. Local overlays of `appropriate low-fusing-poin-t link-severing metals may take the place of the aforementioned overall plating of the link, as will be shown below more in detail. Plates 6 have areas 6a covering a rst plurality of points of reduced cross-section and 'thus substantially separating said first plurality of points of reduced cross-section from the pulverulent arcquenching filler 2. The structure shown in FIGS. l and 2 is `a fuse wherein eight points of reduced cross-section 4a are removed by plates 6 from immediate action of the arc-quenching illler. lnsulating plates 6 further have cut-out areas 6b exposing a second plurality of pointsjof reduced cross-section to the direct action of pui-verulent arc-quenching filler 2. The structure shown in FIGS. l and 2 is a fuse wherein seven points of reduced cross-section 4a are exposed to the direct action of pulverulent arc-quenching -iiller 2. Areas 6a shielding points of reduced cross-section 4a of link 4 from the immediate action of filler 2 alternate with cut-out areas 6b exposing points of reduced cross-section of link e to the direct action of ller 2. Some of the areas 6b of plates 6 exposing the aforementioned second plurality of points cf reduced cross-section are bounded to both sides thereof by contiguous said areas `6a covering said first plurality of points of reduced cross-section. Eyelets 7 project transversely lthrough plates 6 and fuse link 4 at the point of the latter where the center perforations thereof are located. The diameter of eyelets 7 is substantially less than lthe diameter of the center perforations through which they project, thus providing a limited freedom of relative motion between plates -6 and ribbon link 4.

On occurrence of dangerous overload currents one or more points of reduced cross-section at, or close to, the center region of link 4 reach the fusing point of the plating metal thereof. This results in a metallurgical reaction with the Ibase metal of link 4 causing link 4 to be severed at one or several serially related points thereof. As shown in the aforementioned U.S. Patent 3,020,372 to F. I. Kozacka the presence of plates 6 tends to result in increased delay times on occurrence of relative small overloads of inadmissible duration.

0n occurrence of short-circuit currents, or of fault currents of a similar nature, arcs are instantly kindled at all points of reduced cross-section. As shown in the laforernentioned US. Patent 2,964,604 to P. I. Jacobs, lr. et al., the arc voltage across the breaks not covered by plates 6 has a characteristic different from that across the breaks covered by plates 6. This results in a stabilization of the total larc voltage, as also explained in that patent.

A novel feature of the structure of FIGS. 1 and 2 not occurring in the aforementioned paten-ts consists in that a more effective limitation of burn-back is achieved in the structure of FiGS. 1 and 2. ln addition thereto in the structure of FIGS. 1 and 2 a more limited decay of the arc voltage is achieved at the points of reduced crosssection exposed to the immediate action of the arc quenching filler. This will be more apparent from a consideration of FIGS. and 6 wherein the same reference characters have been applied as in FTGS. l and 2 to designate like parts. lt is apparent from FIGS. 1, 2, 5 and 6 that plates 6 form barriers limiting the burn-back in opposite directions of the points of reduced cross-section da exposed to the immediate action of arc-quenching filler 2. In FiG, 5 the directions of burn-back from a point of reduced cross-section has been indicated by arrows B. As the arc gap grows the arc terminals formed on link d move closer to the edges of plates 6 bounding the areas 6b. Since plates 6 are made of an insulating material evolving gas under the heat of electric arcs, the movement of the arc terminals on link l toward the `aforementioned edges results in the generation of blasts of gas which have been diagrammatically indicated by the arrows R. These blasts of gas are of opposite direction and highly turbulent. The turbulence thus established tends to increase, or stabilize, the arc voltage during the last phases of the interrupting process when arc voltage tends to decay in the center region of the arc due to prolonged heat generation at this particular point. This will become more apparent from a consideration of FIG. 7 given below. :Because of the head-on collision of the aforementioned two blasts of gas R the resulting gas ilow tends to move radially outwardly in casing 1l, i.e. away from the axis region thereof, or the region where the temperature tends to be highest and where cooling is most necessary for achieving the desired degree of dielectric recovery. l

Referring now to FlG. 7, that ligure shows arc voltage in terms of volts plotted against fthe Width of an arc gap formed in a fuse and expressed in inches. At the time of arc initiation the gap width is zero. 11 milliseconds after kindling of an iarc the arc length is AL1 and the voltage across the arc gap is AV1. At this point of time the average voltage gradient is in the order 100 volts/inch. 44 milliseconds after kindling of an arc the length thereof has increased to AL2 and the arc voltage has increased to AV2. At this point of time the average voltage gradient is in the order of 50 volts/inch. lt is apparent from FIG. 7 that the arc voltage and its gradient undergo a dramatic decay at the region of arc initiation. A gain in arc voltage overcompensating the decay thereof in the region of arc initiation is achieved by elongation of the arc and the motion of the terminals thereof into relatively cool arc quenching filler. FIG. 7 refers to an experiment made with a copper ribbon link in a D.C. circuit, the entire length of the link being exposed to the immediate action of the pulverulent arc-quenching ller.

In a structure of the type shown in FlGS. 1, 2, 5 and 6 the rate of burn-back is decreased and the arc voltage increased and stabilized in comparison to the performance illustrated in FIG. 7. This is due to the presence of a plurality of counterows R of gas at a relatively large number of iarc gaps or points of break. On occurrence of maior fault currents the structure of FIGS. 1 and 2 forms fifteen series breaks, and on seven out of these fifteen series breaks arc-quenching counterilows R of gas are generated. The fact that fuse structures of the type shown in FIGS. 1, 2, 5 and 6 generate relatively high relatively stable arc voltages with fuse links of relatively short length makes it possible to manufacture compact fuses for relatively high circuit voltages, i.e. voltages above 600 volts up to several kilovolts, having a relatively high current carrying capacity.

Thus FIGS. 3 and 4 illustrate an extremely compact fuse structure having a voltage rating of 1000 volts and a current rating of 500 amps. embodying this invention. FIGS. 3 and 4 have been drawn on a 1:1 scale and thus show that fuse structures embodying this invention are much more compact than any prior art fuse structures having the same voltage rating and current rating.

ln FiGS. 3 and 4 the same reference numerals as in FIGS. l, 2, 5 and 6 have been applied to indicate like parts, however, with prime signs added to the respective reference numeral. rl`l1us l' has lbeen applied to indicate a tubular casing of a synthetic-resin-glass-cloth laminate containing an arc-quenching filler body 2' of quartz sand. rerminal plugs 3' aire press-fitted into casing l and tirmly held therein by transverse steel pins 3b'. Pins 3b' are angularly displaced and the number of pins 3b' depends 'upon strength requirements, their number varying between 4 and 6 per plug 3'. Each plug 3' is provided with one or more bores extending in a direction longitudinally of casing l' and each is closed by a resilient sheet metal cap S'. The aforementioned bores facilitate tilting of casing 1l with the pulverulent arc-quenching iiller 2' after casing 1' has ybeen closed by plugs 3'. Each plug 3 is provided with a radial groove into which a blade Contact 9 is inserted. Blade contacts 9' may be brazed to plugs 3'. Blade contacts l define a .median plane of the fuse structure of FIGS. 3 and 4. A lirst pair 4a of radially inner chevron-shaped rib-bon fuse links is housed inside or casing i and arranged symmetrically in regard to the aforementioned `median plane. Each plug 3 has four parallel grooves 3c' on the axially inner surface thereof and the ends of fuse links 4c are inserted into the r; ially inner two of said four `grooves 3c'. The fuse structure of 3 and 4 further comprises a second pair of radially outer chevron-shaped ribbon fuse links Lib' arranged symmetrically in regard to the same median plane of casing Il as the first pair of fuse links 4a. Fuse links 1lb' Ihave ends inserted into the radially outer two of said four ygrooves 3c'. Grooves 3c' are lled with soft solder to establish current paths of low impedance between links dal', 4b' and plugs 3'. Each of links 4a', 4b have five transverse lines A', B', C', D', E' of perforations. Each of the aforementioned lines of perforations comprises `four circular perforations and two semi-circular perforations and each line of perforations A', B', C', D', E forms, in effect, a transverse region of reduced crosssection. Reference character C' has been applied to indicate the center lines of perforations of links 4a', 4b'. An overlay 9" of a low-fusing-point link-destroying metal, e.g. tin, is arranged on each fuse link da' and 4b' in close proximity to the center line C' of perforations. The overlay of each fuse link 4a', 4b' is preferably arranged in accordance with the teachings of U.S. Patent 2,988,620 to F. l. Kozacka, Time-Lag Fuses, lune 13, 1961. Reference may be had to that patent for further details regarding the arrangement of overlays 9" on fuse links da', 4b' and the operation of a fuse structure cornprising fuse links and overlays of the kind disclosed and claimed in the aforementioned U.S. patent. One significant difieren-ce between the structure of U.S. Patent 2,988,620 and that sholwn in FIGS. 3 and 4 resides in the fact that the latter comprises a radially inner pair or layer of chevron-shaped fuse links 4:1' and a radially outer pair or layer of chevron-shaped fuse links 4b. This two or double layer arrangement is conducive to increased time lags under otherwise `identical conditions. The structure of FlGS. 3 and 4 further comprises two pairs of insulating plates 6' on each fuse link da', 4b' sandwiching the lines B', D' of perforations immediately adjacent to the `center lines C' of perforations. Thus the lines B', D' of perforations are removed from the immediate action of quartz filler Z' but the remaining three lines A', C', E' of perforations are exposed to the immediate action of the Iarc-quenclling quartz filler 2'. Plates 6' are maintained in position by eyelets 7' having a smaller diameter than the circular perforations in links 4a', db' projecting through plates 6' and links 4a 4b' at points where the latter are perforated.

On occurrence of dangerous overload currents the regions deiined by the center lines C' of perforations have the highest temperature within the entire struct-ure. To be more specilic, the temperature of the regions defined by the center lines C' of links da is highest and the ternperature of the regions delined by center lines C of links 4b' slightly less. As a resul-t, links da' will form initial breaks at the regions defined by their center lines C' and thus links 4b `are caused to carry all of, or substantially all of, the overload current. This results in rapid fusion of the overlays 9" on links 4b' and consequent formation of center breaks on links 4b'. -As shown in U.S. Patent 3,020,372 to F. J. Kozacka the presence of fuse link sandwviching plates 6, preferably made of a synthetic-resin- -glass-clcth laminate, e.g. a melamine-resin-glass-cloth laminate, on a fuse link having a low-using-point linkdestroying metal overlay results in increased delay times on occurrence of overload currents.

On occurrence of short-circuit currents, or of fault currents of a similar nature, arcs are instantly kindled at the regions of all five lines A', B', C', D', E' of perforations. As shown in the aforementioned Patent 2,964,604 to P. C. Jacobs, lr., et al. the voltage versus time cha-racteristie of the yarcs at the areas of links 4a', lb not covered by plates `6' is dillerent from the voltage versus time characteristics of the arcs at the areas of links la 4b' covered by plates `6i'. This result-s in a stabilization of the arc voltage, as also more fully explained in the aforementioned patent to P. C. Jacobs, Jr., et al.

The arcs kind-led at the regions of center lines C' of perforations cause burn-back of the fuse links in opposite axial directions. This burn-back is limited and decelerated by opposite blasts of gas evolving from plates 6', 'as more fully explained in connection with FIGS. 5 and 6. The 4arcs kindled at the regions of the axially outer lines A', E of perforations cause also burn-back in opposite directions, the axially inward 'burnaback of t-he arcs being limited, and deceler-ated, by opposite axially outwardly directed blasts oi gas evolving from the axially outer edges of plates 6'. The axially outer burn-back of the arcs kindled at the regions of the axially outer lines A', E' ot perforations is not limited by insulating barrier means evolving arc-quenching gases under the heat of arcs, this being unnecessary at voltage ratings up to the order of 1G00 volts at current ratings up to the order of 500 amps.

lt `will be understood that I have illustrated and described herein two preferred embodiments of my invention, and that various alterations may be made therein without departing from the spirit and scope of the appended claims.

I claim as my invention:

1. An electric fuse comprising:

a tubular casing of insulating material;

a pulverulent arc-quenching iiller within said casing;

a pair or Itermina-l elements closing the ends of said casing;

a ribbon fuse link of a relatively high fusing point metal `arranged inside said casing `and deiining serially related points of reduced cross-section conductively interconnecting said pair of terminal elements;

a plurality of insulating barriers of a material evolving gas under the heat of electric arcs supported by said link, each of said plurality of insulating barriers being formed by a pair of insulating plates sandwiching alternate points of reduced cross-section of said link and exposing points of reduced cross-section of said link situated between contiguous insulating barriers to the action of said arc-quenching filler, each of said plurality of insulating barriers being arranged between a pair of points of reduced cross-section of said link exposed to the action of said arc-quenching filler, and each of said plurality of insulating barriers limiting the burn-.back-paths in opposite directions of a pair of said points of reduced cross-section of `said link exposed to the action of said arc-quenching filler.

An electric fuse comprising:

tubular casing of insulating material;

pulverulent Iarc-quenching iiller Within said casing;

pair of terminal elements closing -the ends of said easa ribbon fuse link of a metal having a relatively high fusing point arranged inside said casing and delining serially related points of reduced across-section conductively interconnecting said pair of terminal elements; and 1 a pair of insulating plates of Ia material evolving gas under the heat of electric arcs sandwiching said link along a preponderant portion of the entire length thereof, said pair of plates having a plurality of areas each covering one of a lirst plurality of points of reduced cross-section and thus substantially separ-ating said lirst plurality of points of reduced crosssection from said rare-quenching liller, said pair of insulating plates further having a plurality of cut out areas eaclh exposing one of a second plurality of points of reduced cross section to the direct action of said arc-quenching liller, each of said plurality of areas covering one of said rst plurality of points of reduced cross-section being arranged between a pair of said plurality of areas each exposing one of said second plurality of points of reduced cross-section; and opposite edges of each of said plurality of areas covering one of said iirst plurality of points of reduced cross-section limiting burn-back in opposite directions of `a pair of said second plurality of points of reduced cross-section.

3. Ain elec-tric fuse comprising:

a tubular easing of insulating material;

la pulverulent quartz liller within said casing;

`a pair of terminal elements closing the ends of said casa ribbon fuse link of a metal having a relatively high fusing point arranged inside said casing and delining serially related transverse lines of perfor-ations conductively interconnecting said pair of terminal elements; and

a pair of insulating plates of a synthetic resin-glasscloth laminate sandwiching said fuse link along a preponderant portion of the entire length thereof, said pair of plates having 1a plurality of are-as eacl'i covering one of `a lfirst plurality of said lines of perforations and thus substantially separating said lirst plurality of lines of perforations from said quartz filler, said pair of plates further having a plurality of cut-out areas each exposing one of a second plurality of said lines of perforations to `the direct action of said quartz filler, each of said plurality of areas covering one of said first plurality of points of reduced cross-section being arranged between a pair of said plurality of areas exposing one of said second plurality of points of reduced cross-section, and opposite edges of each of said plurality of areas cover-ing one of said tirst plurality of points of reduced cross-section limiting the burn-back-path in opposite directions of a pair of said second plurality of points of reduced cross-section.

4. An electric fuse comprising:

-a tubular casing of insulating material;

a pulverulen-t quartz liller within said casing;

a pair of .terminal elements closing the ends of said casing;

a ribbon fuse link of a metal having a relatively high fusing point arranged inside said casing and delining serially related transverse lines of circular perforations conduct-ively interconnecting said pair of terminal elements;

a pair of plates of an organic insulating material having ia Width exceeding the width of said fuse link `sanclwiching said fuse link along a preponderant portion of the entire length thereof, said pair of plates having Iarea covering a first plurality of said lines of perforations and thus substantially separating said first plurality of lines of perforations from said quartz filler, said pair of plates further having cntout portions expo-sing the entire width of said fuse link to the direct action of said filler at a second plurality of said lines of perfonations, each of said second plurality of lines of perfonations alternating with one of said rst plurality of lines of perforations; ,and fastener means having a smaller diameter than said circular perforations projecting through said pair of plates and projecting through circular perforations of said fuse link for securing said pair of plates to said fuse link.

5. An elec-tric time-lag fuse for circuits having a relatively high voltage comprising:

a twbular casing of a synthetic-resin-gliass-cloth laminate;

a pulverwlent quartz iller within said casing;

a pair of terminal elements closing the ends of said casing;

a first pair of radially inner chevron-shaped ribbon fuse link-s inside `said casing arranged symetrically in regard to a median plane of said casing and conductively interconnecting said pair of terminal elements;

a second pair of radially outer chevron-shaped ribbon fuse inks inside said easing arranged symmetnically in regard to the `same median piane of said casing as said first pair of fuse links and conductively interconnecting said pair of terminal elements;

:each of said iirst pair of fuse links and each of said second pair of fuse links having a plurality of transverse lines of perforations, said a plurality of lines of perforations of each of said first pair of fuse links and of each of said second pair of fuse links including a center line of perforations;

an overlay of a low-fusingpoint llink-destroying metal arranged on each of said tirst pair of fuse links and on each of said second pair of fuse links in close proximity of said center line of perforations; and

insulating plate means supported by each of said rst pair of fuse links and by each of said second pair of fuse links sandwiching the lines of perforations thereof immediately adjacent said center line of perforations and thereby substantially separating the sandwiched lines of perforations from said quartz filler, said insulating plate means sandiwiching alternate transverse lines of perforations of each of said first pair of iuse links and of each of said second pair of fuse links and exposing alternate transverse lines of perforations of each of said first pair of fuse links and of each of said second pair of fuse llinks and of each of said second pair of fuse links to the immediate action of said quartz filler.

References Cited in the tile of this patent UNITED STATES PATENTS 2,837,614 fFister lune 3, 1958 2,858,396 Sugden Oct. 28, 1958 2,876,312 Frederick Mar. 3, 1959 2,892,061 Kozacka lune 23, 1959 2,964,604 Jacob et al. Dec. 13, 196() 3,020,372 Kozacka lFeb. 6, 1962 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent N0 3qII3v195 December 3.l 1963 Frederick J Kozaeka It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corres ted below Column 8i line 23Er strike out "and of each of said second pair of fuse Inks"; the last page of the Letters Patent containing duplicates of columns 5 c 7 and R should be cancelled in its entirety.

Signed and sealed this 12th day' of May 1964.

(SEAL) Attest:

ERNEST W SWIDER EDWARD .L BRENNER Attesting Officer Commissioner of Patents 

1. AN ELECTRIC FUSE COMPRISING: A TUBULAR CASING OF INSULATING MATERIAL; A PULVERULENT ARC-QUENCHING FILLER WITHIN SAID CASING; A PAIR OF TERMINAL ELEMENTS CLOSING THE ENDS OF SAID CASING; A RIBBON FUSE LINK OF A RELATIVELY HIGH FUSING POINT METAL ARRANGED INSIDE AND CASING AND DEFINING SERIALLY RELATED POINTS OF REDUCED CROSS-SECTION CONDUCTIVELY INTERCONNECTING SAID PAIR OF TERMINAL ELEMENTS; A PLURALITY OF INSULATING BARRIERS OF A MATERIAL EVOLVING GAS UNDER THE HEAT OF ELECTRIC ARCS SUPPORTED BY SAID LINK, EACH OF SAID PLURALITY OF INSULATING BARRIERS BEING FORMED BY A PAIR OF INSULATING PLATES SANDWICHING ALTERNATE POINTS OF REDUCED CROSS-SECTION OF SAID LINK AND EXPOSING POINTS OF REDUCED CROSS-SECTION OF SAID LINK SITUATED BETWEEN CONTIGUOUS INSULATING BARRIERS TO THE ACTION OF SAID ARC-QUENCHING FILLER, EACH OF SAID PLURALITY OF INSULATING BARRIERS BEING ARRANGED BETWEEN A PAIR OF POINTS OF REDUCED CROSS-SECTION OF SAID LINK EXPOSED TO THE ACTION OF SAID ARC-QUENCHING FILLER, AND EACH OF SAID PLURALITY OF INSULATING BARRIERS LIMITING THE BURN-BACK-PATHS IN OPPOSITE DIRECTIONS OF A PAIR OF SAID POINTS OF REDUCED CROSS-SECTION OF SAID LINK EXPOSED TO THE ACTION OF SAID ARC-QUENCHING FILLER. 